Various methods and apparatus are known for producing laser radiation which is characterized by: a) high power; b) diffraction-limited divergence; and c) narrow bandwidth. Methods and apparatus which provide all three characteristics simultaneously are usually costly and complicated, and often require multiple laser sources.
Stable laser cavities, for example, are used to provide radiation having low spatial divergence and narrow bandwidth. Low spatial divergence is obtained by restricting the laser aperture to operate in the lowest transverse mode of the stable cavity. For near infrared or shorter wavelengths, cavity apertures with diameters on the order of 1 millimeter (mm) or less are required. As a result, the power available from a stable laser cavity is limited, often to values that are orders of magnitude less than values that would be obtainable if the full aperture could be used. The bandwidth of stable cavities is usually restricted by using a conventional dispersing element, i.e., a grating, a prism or a Fabry-Perot etalon, in the cavity to spread the radiation angularly according to wavelength. Narrow-bandwidth operation is then obtained by restricting the angular acceptance of the cavity. This operation is compatible with a mode-restricting aperture used for limiting the spatial divergence that was described above, but is incompatible with high power operation.
High power laser sources are obtainable with narrow bandwidths and low divergence radiation using laser amplifiers downstream of the oscillator. Such systems are the primary source for laser radiation with all three desired characteristics, but the multiple lasers required by these systems increase both the size and cost of the system. In addition, the complexity of the optical train required to match the output of one stage into the input of the next stage increases. In some pulsed laser systems, jitter in timing between the various stages can reduce system reliability.
Unstable resonators provide an alternative approach to obtaining high power, low divergence laser radiation. In unstable resonators, the laser radiation fills a relatively large diameter cavity, allowing operation at high power levels while restricting the divergence of the generated laser radiation to a low value, usually near the diffraction limit for a suitably designed system, as is well known to arise from the properties of the transverse modes of unstable resonators. Some success has been achieved in frequency narrowing the laser radiation from unstable resonator cavities using gratings. This technique works best with lasers having sharp line structure, e.g., molecular lasers. For example, selection of a single line in HF and CO.sub.2 lasers by insertion of a grating into a conventional unstable resonator cavity is known.
In lasers having broad-band continuous gain distributions, i.e., excimer and dye lasers, insertion of a grating in an unstable resonator laser cavity does not provide sufficient spectral discrimination for narrow-band operation. The conventional unstable resonator laser cavity is fundamentally incompatible with the requirements of frequency narrowing elements used in restricting the bandwidth, especially when extremely narrow linewidths are desired. The modes of an unstable resonator laser cavity require that divergence of the laser radiation inside the cavity alternate between high and low values on alternate passes through the cavity, while frequency narrowing elements work best with collimated radiation. As a result, it is not possible to use angular discrimination to restrict the bandwidth of the laser radiation as is done in stable cavities. Thus, although unstable resonator laser cavities are the configuration of choice for providing high power, low divergence laser radiation they are usually not compatible with a simultaneous requirement for narrow bandwidth.
An unstable resonator laser cavity employing a telescopic full cavity ring, in which the gain medium and the ring form a continuous loop and the magnification is achieved by a telescope within the ring, is also known. This cavity has extensive collimated regions that offer potential for frequency narrowing. However, the beam passes through the gain medium only once on each cavity round trip. As a result, this type of cavity generally requires a large number of cavity transits to reach threshold and can work only with lasers that have gain media with a combination of high gain and long lifetime.
An unstable resonator laser cavity developed by some of the inventors of the present invention is disclosed in U.S. Pat. No. 4,868,515, which discloses all three desirable characteristics of high power, low divergence and narrow bandwidth by employing an asymmetric feedback ring. The invention represents a substantial improvement in the state of the art because the feedback ring provides a section of the cavity that contains only collimated laser radiation, thus allowing optimal use of frequency narrowing devices, e.g., Fabry-Perot etalons. At the same time, the length of the feedback ring can be kept to a minimal value, and the laser radiation makes two passes through the gain medium on each cavity transit, thus overcoming the limitations of the cavity with the full telescope ring. As a result, this type of cavity works with all types of lasers, including the class of lasers in which the gain medium has a limited gain or lifetime, such as electric-discharge, rare-gas-halide excimer lasers, with which the resonator with the full cavity ring would not work.